Optical molecular imaging is an attractive modality that has been employed recently in many aspects of biomedical research aiming at using light to detect cellular and molecular events in vivo. This targeted imaging technique largely relies on near-infrared (NIR) dyes that emit light in the NIR window (700-900 nm). Imaging in this range is of importance because of the increased tissue penetration and reduced absorption by physiologically abundant molecules such as hemoglobin (600 nm) and water (>1200 nm) compared to other wavelengths. Optical dyes operate by absorbing energy at one wavelength but the reemitting light at a longer wavelength. The difference in the absorbed and emitted wavelengths (the Stokes shift) is an important feature of the dye because it affects how well the dye can be detected reliably. Most current NIR dyes have rather poor performance because of their Stokes shift is small (30 nm).
Without being bound by theory, part of the explanation is the rigid structure of the current dyes that have unsaturated polymethine backbones. This affects on the ability to separate the excitation from the emission photons discretely, thus limiting the ability to image multiple wavelengths and causes tremendous cost in the effort to create steep-edge bandpass filters in detection to cope with this problem. The present inventors have used a novel approach to generate large Stokes shift NIR dyes using a conventional intramolecular charge transfer mechanism.
Most near-infrared dyes currently available in the market have very narrow Stokes shift (˜30 nm). This may result in cross-talk during imaging processing. On the other hand, embodiments of the present invention have larger Stokes shift (at least about 100 nm, and in embodiments of the invention, about 150 nm); in addition, the excitation spectrum is very wide, which allows excitation of the dye in the visible range while collecting the emission photons in the near-infrared window. This feature allow researchers to image multiple wavelengths corresponding to several molecular events in one target using these dyes.